Pumbs

Pump Training

There is more involved to a pump than simply starting the motor and pumping liquid. Here we have outlined some of the science behind pumping, how to read a pump curve and how to select a pump for your jobsite.

Bypass Pump Training Workshops

Rain for Rent offers bypass training workshops in many areas of North America. We bring our jobsite experience and pumping expertise and will train you and your employees about bypasses and centrifugal pumps.

Most workshops include:

Classroom training and hands-on pump setup training.

You may be able receive “contact hours” (continuing education credits) through their state chapter of the Water Environment Federation (WEF). This varies by state.

Breakfast coffee and donuts/bagels and a catered lunch.

We'll bring a pump or two and train on proper pump set up and use, including troubleshooting and centrifugal pump maintenance.

Centrifugal Pump Theory

The impeller spins & throws water out. -like swinging a bucket of water above your head and staying dry or throwing clay on a potter's wheel and wearing it.

Low pressure is formed in the inlet. - the lower the pressure, the higher the pump can "suck"

Atmospheric pressure pushes more water in.

It is this simple - this is the major part of pump theory. Understand it, and net positive suction head (NPSH) is easy.

Pumps don't suck.

In fact, nothing sucks. Can you name something that does ? Centrifugal Pump Theory also explain the workings of several things in our world:

Breathing

Flight

Wind

Carburetors

Vacuum cleaners

Pump Terms

Head

Centrifugal pump curves show 'pressure' as head, which is the equivalent height of water with S.G. = 1. This makes allowance for specific gravity variations in the pressure to head conversion to cater for higher power requirements. Positive Displacement pumps use pressure (ie; psi or kPa) and then multiply power requirements by the S.G.

Static Head

The vertical height difference from surface of water source to center line of impeller is termed as static suction head or suction lift ('suction lift' can also mean total suction head). The vertical height difference from center line of impeller to discharge point is termed as discharge static head. The vertical height difference from surface of water source to discharge point is termed as total static head.

Net positive suction head - related to how much suction lift a pump can achieve by creating a partial vacuum. Atmospheric pressure then pushes liquid into pump. A method of calculating if the pump will work or not.

A number which is the function of pump flow, head, efficiency etc. Not used in day to day pump selection, but very useful as pumps with similar specific speed will have similar shaped curves, similar efficiency / NPSH / solids handling characteristics.

Vapor Pressure

If the vapor pressure of a liquid is greater than the surrounding air pressure, the liquid will boil.

Viscosity

A measure of a liquid's resistance to flow. ie: how thick it is. The viscosity determines the type of pump used, the speed it can run at, and with gear pumps, the internal clearances required.

Friction Loss

The amount of pressure / head required to 'force' liquid through pipe and fittings.

Reading Centrifugal Pump Curves

The curve consists of a line starting at "shut head"(zero flow on bottom scale / maximum head on left scale). The line continues to the right, with head reducing and flow increasing until the "end of curve" is reached, (this is often outside the recommended operating range of the pump).

Flow and head are linked, one can not be changed without varying the other. The relationship between them is locked until wear or blockages change the pump characteristics.

The pump can not develop pressure unless the system creates back pressure (ie: Static (vertical height), and /or friction loss). Therefore the performance of a pump can not be estimated without knowing full details of the system in which it will be operating.

The above pump curve sample image shows:

Three performance curves ( various impellers or speed).

Curves showing power absorbed by pump (read power at operating point.) Power absorbed by pump is read at point where power curve crosses pump curve at operating point.However this does not indicate motor / engine size required. Various methods are used to determine driver size.

Centrifugal Pump Operating Range

All types of pumps have operational limitations. This is a consideration with any pump whether it is positive displacement or centrifugal. The single volute centrifugal pump ( the most common pump used worldwide) has additional limitations in operating range which, if not considered, can drastically reduce the service life of pump components.

Best Efficiency Point is not only the operating point of highest efficiency but also the point where velocity and therefore pressure is equal around the impeller and volute. As the operating point moves away from the Best Efficiency Point, the velocity changes, which changes the pressure acting on one side of the impeller. This uneven pressure on the impeller results in radial thrust which deflects the shaft causing:

Excess load on bearings.

Excess deflection of mechanical seal.

Uneven wear of gland packing or shaft / sleeve.

The resulting damage can include shortened bearing / seal life or a damaged shaft . The radial load is greatest at shut head.

Outside the recommended operating range damage to pump is also sustained due to excess velocity and turbulence. The resulting vortexes can create cavitation damage capable of destroying the pump casing, back plate, and impeller in a short period of operation.

When selecting or specifying a pump, it is important not to add safety margins or base selection on inaccurate information. The actual system curve may cross the pump curve outside the recommended operating range. In extreme cases the operating point may not allow sufficient cooling of pump, with serious ramifications!

The best practice is to confirm the actual operating point of the pump during operation (using flow measurement and/or a pressure gaug ) to allow adjustment (throttling of discharge or fitting of bypass line) to ensure correct operation and long service life.

Selecting a pump

To ensure the correct pump is selected for your application the following details are required. If you can not supply some of the information, just ask for help from Rain for Rent, we can assist in identifying your requirements.

System Curves

Draw a chart with flow on bottom scale and head on left scale. Estimate scale required based on size of existing pump, or guess maximum flow expected - example shows max flow as 100 L/S and max head as75m - sometimes you just have to guess to get started.

Mark static head. 17m at zero flow. Note: 'Demand' pressure, ie: sprinklers etc, should be added at each flow point, or for approximate figures can be added to static head.

You have completed the System Curve. The Curve may have to be extended to suit higher flow pumps.

The pump operating point is where a pump curve crosses the system curve. Draw as many pump curves over the system curve as you like, to see where different pumps will operate, or draw system curve over pump curve.

If pump curve does not cross system curve, the pump is not suitable.

If the pump curve crosses the system curve twice, then the pump will be unstable and is not suitable.

Pumps Operating in Series and Parallel

When operating pumps in parallel or in a series, there are more complex issues to consider.

Series applications: consider the pressure rating of pump, shaft seal, pipework and fittings. Placement is critical to ensure both pumps are operating within their recommended range and will have a constant supply of water. Drawing a curve for 2 or more pumps is simple, draw 1st pump curve then draw 2nd curve, adding the head each pump produces at the same flow. More curves can be added in the same way.

Parallel applications: confirm suitability of pumps by drawing a system curve (often 2 pumps will only deliver slightly more than one pump due to excessive friction loss. Also you can confirm that pump operation will be within its recommended range.). Non return valves are required especially if one pump operates alone at times.Dissimilar pumps or pumps placed at different heights requires special investigation. Drawing a curve for 2 or more pumps is simple, draw 1st pump curve then draw 2nd curve, adding the flows each pump delivers at the same head. More curves can be added in the same way.

What causes pump cavitation?

There are two main causes to cavitation.

NPSH (r) EXCEEDS NPSH (a) Due to low pressure the water vaporizes (boils) and higher pressure implodes into the vapor bubbles as they pass through the pump causing reduced performance and potentially major damage.

Suction or discharge recirculation The pump is designed for a certain flow range, if there is not enough or too much flow going through the pump, the resulting turbulence and vortexes can reduce performance and damage the pump.

NPSH(r) is the Net Positive Suction Head Required by the pump, which is read from the pump performance curve. Think of NPSH(r) as friction loss caused by the entry to the pump suction.

NPSH(a) must exceed NPSH(r) to allow pump operation without cavitation. It is advisable to allow approximately 1 metre difference for most installations. The other important fact to remember is that water will boil at much less than 100 deg C if the pressure acting on it is less than it's vapor pressure, ie water at 95 deg C is just hot water at sea level, but at 1500m above sea level it is boiling water and vapor.

The vapor pressure of water at 95 deg C is 84.53 kPa, there was enough atmospheric pressure at sea level to contain the vapor, but once the atmospheric pressure dropped at the higher elevation, the vapor was able to escape. This is why vapour pressure is always considered in NPSH calculations when temperatures exceed 30 to 40 deg C.

Affinity Laws of Centrifugal Pumps

If the speed or impeller diameter of a pump change, we can calculate the resulting performance change using affinity laws.

The flow changes proportionally to speed. Double the speed / double the flow.

The pressure changes by the square of the difference. Double the speed / multiply the pressure by 4.

The power changes by the cube of the difference Double the speed / multiply the power by 8.

Remember:

These laws apply to operating points at the same efficiency.

Variations in impeller diameter greater than 10% are hard to predict due to the change in relationship between the impeller and the casing.

I know you are thinking "what does this have to do with anything"?, but if you can understand these 'laws' then you can make rough estimates without having to find full information, which might not be available anyway.

it might go something like this:

Boss: "Hey Joe, put this new pulley on that pump"

Joe: "But that will speed the pump up by about 10 % which increases the power by a third, do you reckon the motor will handle it ?"

For rough calculations you can adjust a duty point or performance curve to suit a different speed. NPSH (r) is affected by speed / impeller diameter change = DANGER!

Pump Troubleshooting

Only one thing is a better troubleshooting tool than pressure & vacuum gauges...that is: readings from pressure & vacuum gauges taken prior to the problem. ie: monitoring gauge readings will help diagnose pump and system problems quickly, by reducing the possible causes.

Flow measurement would allow full diagnosis of pump performance but is sometimes expensive and usually not possible (Cheap versions include: V notch weir, measuring discharge from horizontal pipe, & timing of filling / emptying). System curves can be used in evaluating results.

Here is a troubleshooting table for typical pump symptoms and possible causes.